This invention relates to apparatus and methods for monitoring the composition of a gaseous environment. More particularly, this invention is concerned with apparatus and methods for continuously sampling gases and continuously monitoring the gases for total reduced sulfur content.
Many industrial processes use fuels and chemicals which produce gases containing reduced sulfur compounds in gaseous form. Many of these reduced sulfur gases, such as hydrogen sulfide, methyl mercaptan, dimethyl sulfide and dimethyl disulfide are not only malodorous but are welfare-related pollutants. Environmental restrictions, accordingly, limit the total reduced sulfur content which gases vented to the atmosphere may contain. Industrial plants are necessarily careful to minimize the amount of reduced sulfur gases which are produced, such as by use of low sulfur-containing fuels, substitution of raw materials and by installation of gas treating equipment suitable for withdrawal of gaseous reduced sulfur compounds. In spite of all these efforts it is essential that the gas vented to the atmosphere be monitored continuously to determine that the gas standards for purity are met.
To properly monitor a waste gas for total reduced sulfur content, a sample of the gas must first be taken, usually from a flue or stack, treated as appropriate to prepare the sample for analysis by the test procedure to be employed, and then analyzed. The gas sample is usually hot with a high water content, such as 30-40% by volume, and is generally taken quite far from the location where it is analyzed. Before the gas sample is analyzed it cools, water in the sample condenses and water soluble reduced sulfur gases in the sample are adsorbed in the water thereby leaving a gas sample for analysis having less reduced sulfur gases than the initial or raw sample.
A further problem in conditioning or preparing a sample of gas to be tested is in treating it to remove sulfur dioxide which, if not removed, will lead to an inaccurate analysis for total reduced sulfur. Present methods pass the entire gas sample through a comparatively large volume of scrubbing liquid which removes the sulfur dioxide on a batch basis. This procedure requires use of a large volume of scrubbing liquid to handle gas sampling over a substantial period of time. The time needed for the gas and scrubbing liquid to reach equilibrium when a large volume of each is used, to obtain an accurate analysis, requires that the total time needed for the test extend for hours.